The present invention relates to the technical field of an image recording method and an image recording apparatus, and more particularly, to an image recording system using a two-dimensionally disposed light modulator, for example, a micro mirror array, a two-dimensional mirror array device such as a Digital Micromirror Device(trademark), and the like.
Mainly used in a digital image exposure system utilized in various types of printers is a so-called laser beam scan exposure (raster scan) for two-dimensionally exposing a recording medium with a laser beam modulated in accordance with an image to be recorded by deflecting the laser beam in a main scanning direction while relatively moving the recording medium and an optical system in an auxiliary scanning direction perpendicular to the main scanning direction.
In contrast, recently, various types of digital image exposure systems have been proposed which use two-dimensional spatial light modulators (2DSLM) such as a liquid crystal display (hereinafter, referred to as xe2x80x9cLCDxe2x80x9d) and the micro mirror array (hereinafter, referred to as xe2x80x9cMMAxe2x80x9d) or the two-dimensional mirror array device (2DMAD) which is commercially available, for example, as a Digital Micromirror Device(trademark) (DMD) manufactured by Texas Instruments Inc. that are utilized as display devices in displays, monitors and so on. In the exposure systems, a recording medium is basically exposed by forming an image displayed by the two-dimensional spatial light modulator on the recording medium.
In particular, the MMA is advantageous in exposure at high speed because the MMA has a modulation speed (response speed) faster than that of the LCD and moreover utilizes light more efficiently than the LCD.
Digital exposure systems using the spatial light modulator are disclosed in, for example, U.S. Pat. No. 5,049,901 and EP 0992350 A1.
The digital exposure systems disclosed in these publications each relate to an image recording apparatus in which an image is formed on a spatial light modulator such as the MMA in the form of an image signal and the image is focused on a recording medium for recording. In the digital exposure systems, the image on the spatial light modulator is moved in synchronism with the movement of the recording medium which is moved in a main scanning direction so that the image can remain stationary on the recording medium. This operation enables image recording of high resolution by obtaining a small recording light spot from an extended light source which has difficulty in narrowing the area.
The principle of an image recording method using the conventional spatial light modulator will be described with reference to FIGS. 12A, 12B and 12C. As shown in FIG. 12A, light impinges on a spatial light modulator 90 and is reflected by a mirror 90a, and the reflected light is imaged onto a recording medium 94 through an optical system such as a lens 92. It is assumed that the recording medium 94 moves at a constant speed as shown by an arrow in FIGS. 12A to 12C. In FIG. 12A, only the mirror 90a is activated and mirrors 90b and 90c are deactivated, and only the light reflected by the mirror 90a is imaged onto the recording medium 94.
Next, when the recording medium 94 slightly moves as shown in FIG. 12B, the mirror 90a in the spatial light modulator 90 is deactivated and only the mirror 90b is activated instead in synchronism with the movement of the recording medium 94, and the same point on the recording medium 94 as shown in FIG. 12A is exposed to the light reflected by the mirror 90b. 
Further, when the recording medium 94 moves as shown in FIG. 12C, only the mirror 90c is activated in the spatial light modulator 90 in synchronism with the movement of the recording medium 94 and images at the same position on the recording medium 94.
As described above, in the illustrated example, the spatial light modulator 90 changes image signaling three times so that each of the mirrors 90a, 90b and 90c exposes the recording medium 94 once, in total three times. As a result, the image is moved in synchronism with the movement of the recording medium 94 so as to remain stationary on the recording medium 94 in the main scanning direction (the moving direction of the recording medium).
However, the conventional exposure systems move an image to be recorded on an recording medium in synchronism with (following) the movement of the recording medium and cause the image to remain stationary in the moving direction (main scanning direction) of the recording medium and describes this technical art, but they describe nothing in particular as to the technical art for moving the image in a direction perpendicular to the moving direction of the recording medium (auxiliary scanning direction) and recording the image two-dimensionally.
Accordingly, when a conventionally known ordinary method is used to move the image in the auxiliary scanning direction, after an image of Nimg s lines is recorded in the main scanning direction, the image is moved relatively with respect to the recording medium by Nimg s pixels in the auxiliary scanning direction, and then the next image of Nimg s lines (second line) is recorded. Thereafter, the operation of recording an image (of Nimg g lines) in the main scanning direction and then moving the image in the auxiliary scanning direction is repeated. Here, Nimg s shows the number of pixels of a special light modulator in the auxiliary scanning direction.
However, in the so-called step and repeat system as described above, the image cannot be continuously moved in the auxiliary scanning direction, and further no image is recorded while the image is being moved in the auxiliary scanning direction. Thus, a problem arises in that the moving time of the image is wasted and a scanning efficiency decreases.
For example, in an outer drum scanning system in which an image is exposed on a recording medium wound around the outside surface of a drum, an image of Nimg s lines is recorded a main scanning direction in one rotation of a drum (first rotation), then an optical system for exposing the recording medium is moved in an auxiliary scanning direction that is the axial direction of the drum while the drum is being rotated once (second rotation), and then the image of the next Nimg s lines is recorded in the next one rotation of the drum (third rotation). Therefore, in this case, a scanning efficiency is reduced to one half, and thus it cannot be said that this system has high productivity because it requires a wasteful time. As described above, the step and repeat system has a problem in that productivity is bad.
Further, in the step and repeat system, the image is not continuously moved, and, for example, an optical system is repeatedly moved and stopped mechanically, thereby vibration is generated. As a result, a problem is arisen in that the quality of an image is degraded and the durability and reliability of an image recording apparatus are decreased.
In order to solve the problem described above, the first aspect of the invention provides an image recording method of recording an image on a recording medium attached around outside surface of a drum rotating at a first constant speed by main-scanning the recording medium in a main scanning direction with a group of light sources that are two-dimensionally disposed and exposing the recording medium as well as auxiliary-scanning the recording medium by moving the group of light sources in an auxiliary scanning direction substantially perpendicular to the main scanning direction, comprising the steps of moving the group of light sources in the auxiliary scanning direction at a second constant speed, and deflecting light from the group of light sources for exposing the recording medium in a direction inclined at a predetermined angle xcex8 with respect to the main scanning direction in synchronism with movement of the recording medium in the main scanning direction and the auxiliary scanning direction with respect to the group of light sources, thereby the image to be recorded on the recording medium is caused to remain stationary in a relative relation to the recording medium in the main scanning direction and in the auxiliary scanning direction.
Preferably, after an image of one frame that is a range capable of being exposed by the group of light sources at a time has been exposed to the recording medium by the group of light sources, the image to be recorded on the recording medium is offset in the auxiliary scanning direction by an integral multiple of a first pixel pitch in the auxiliary scanning direction.
Preferably, when the integral multiple of the first pixel pitch in the auxiliary scanning direction is denoted by Ns, number of pixels of one in the main scanning direction is denoted by Nimg m, a second pixel pitch in the main scanning direction is denoted by Pimg m, and the first pixel pitch in the auxiliary scanning direction is denoted by Pimg s, the predetermined angle xcex8 is represented by the following formula: tan xcex8=(Nsxc3x97Pimg s)/(Nimg mxc3x97Pimg m).
Preferably, movement of an auxiliary scanning position and a deflecting position of an image of one frame that is a range capable of being exposed by the group of light sources at a time is synchronized with the relative movement of the recording medium with respect to the group of light sources that are two-dimensionally disposed during a non-exposure time zone from completion of exposure of the recording medium for one rotation of the drum to start of exposure of the recording medium for the next one rotation of the drum, and the image of one frame exposed to the recording medium by the group of light sources in the next one rotation of the drum is exposed just adjacent to the image of one exposed to the recording medium first by the group of light sources in one rotation of the drum, and images of frames exposed by the group of light sources are arranged substantially at equal intervals.
In order to solve the problem described above, the second aspect of the invention provides an image recording apparatus comprising a drum rotating at a first constant speed, a recording medium attached around outside surface of the drum, a group of light sources that are two-dimensionally disposed for exposing the recording medium in a main scanning direction, an auxiliary scanning drive system for moving the group of light sources in an auxiliary scanning direction substantially perpendicular to the main scanning direction, and a light deflector for deflecting light from the group of light sources, wherein the auxiliary scanning drive system moves the group of light sources in the auxiliary scanning direction at a second constant speed, and the light deflector deflects the light from the group of light sources for exposing the recording medium in a direction inclined a predetermined angle xcex8 with respect to the main scanning direction in synchronism with movement of the recording medium in the main scanning direction and the auxiliary scanning direction with respect to the group of light sources, thereby the image to be recorded on the recording medium is caused to remain stationary in a relative relation to the recording medium in the main scanning direction and in the auxiliary scanning direction.
It is preferable that the image recording apparatus further comprises a device which offsets an image to be recorded on the recording medium in the auxiliary scanning direction by an integral multiple of a first pixel pitch in the auxiliary scanning direction after the image of one frame thereof that is a range capable of being exposed to the recording medium by the group of light sources at a time has been exposed to the recording medium by the group of light sources.
Preferably, when the integral multiple of the first pixel pitch in the auxiliary scanning direction is denoted by Ns, number of pitches of one frame in the main scanning direction is denoted by Nimg m, a second pixel pitch in the main scanning direction is denoted by Pimg m, and the first pixel pitch in the auxiliary scanning direction is denoted by Pimg s, the predetermined angle xcex8 deflected by the deflector is represented by the following formula: Tan xcex8=(Nsxc3x97Pimg s)/(Nimg mxc3x97Pimg m).
It is also preferable that the image recording apparatus further comprises a device which synchronizes movement of an auxiliary scanning position and a deflecting position of an image of one frame that is a range capable of being exposed by the group of light sources at a time with the relative movement of the recording medium with respect to the group of light sources that are two-dimensionally disposed during a non-exposure time zone from completion of exposure of the recording medium for one rotation of the drum to start of exposure of the recording medium for the next one rotation of the drum, wherein the image of one exposed to the recording medium by the group of light sources in the next one rotation of the drum is exposed just adjacent to the image of one exposed first by the group of light sources in one rotation of the drum, and images of frames exposed by the group of light sources are arranged substantially at equal intervals.
Accordingly, it is an object of the present invention made in view of the above problems to provide an image recording method and an image recording apparatus permitting auxiliary scanning to be carried out continuously, that is, capable of increasing productivity in image recording by making a scanning efficiency to substantially 1.0.